The present invention is directed to a phase change material and a method of making the same, and in particular to a phase change material with an inhibitor to acid formation and a method of making the same.
It is recognized in the art that it is desirable to equip a vehicle having an internal combustion engine with a mechanism to store the thermal energy produced during the operation of the engine for use during subsequent start-ups. For example, the stored thermal energy can be used to reduce or eliminate the traditional lag between the supply of and the demand for thermal energy to increase the air temperature of the passenger compartment. Additionally, the thermal energy can be used to preheat the engine so as to reduce the noxious emissions produced.
One such mechanism used to store thermal energy is a latent heat battery. A latent heat battery includes first and second chambers in thermal communication with each other. Coolant from the vehicle coolant system passes through the first chamber, while a material, known as a phase change material, is disposed in the second chamber. During operation of the engine, thermal energy is transferred from the coolant in the first chamber to the phase change material in the second chamber, causing the phase change material to transform between first and second phases, typically solid and liquid phases. When desired, thermal energy is transferred from the phase change material to the coolant as the phase change material transforms between liquid and solid phases.
One common phase change material used in latent heat batteries is a mixture of lithium nitrate (LiNO3) and magnesium nitrate hexahydrate (Mg(NO3)2*6H2O). The hydrolysis of the magnesium nitrate leads to the formation of nitric acid by the reaction:
Mg(NO3)2+2H2O⇄2HNO3+Mg(OH)2
The acid thus formed may corrode the walls of the phase change material chamber, typically made of aluminum.
Although aluminum readily forms a protective layer or passivation film of aluminum oxide (Al2O3) on the surface of the wall, because the aluminum is exposed to an aqueous solution (i.e. the phase change material), the thermodynamic conditions for the maintenance of the passivation film are pH dependent. While passivation in aluminum exposed to an aqueous solution may occur at a pH as low as 4, phase change materials should ideally be buffered to a pH of between 5 and 8.
Currently, lithium phosphate (Li3PO4) is used as an inhibitor in the conventional lithium nitrate:magnesium nitrate hexahydrate phase change material. The lithium phosphate reacts with the magnesium nitrate to form magnesium hydroxide, which can inhibit the formation and evolution of nitric acid. However, the reaction occurs substantially instantaneously, with the magnesium hydroxide precipitating in relatively large clumps. The size of the clumps formed limits the surface area available to interact with the acids formed by the phase change material, which limits the effectiveness of the magnesium hydroxide. Additionally, because of insolubility of magnesium hydroxide in the phase change material and the relative densities of magnesium hydroxide and the phase change material, the clumps promptly settle to the bottom of the chamber where the magnesium hydroxide is relatively inactive, thus further reducing what buffering advantage might be otherwise obtained from the hydroxide.
According to an aspect of the present invention, a phase change material is made by the process including the steps of providing a composition of a metal nitrate and water and adding tetraborate.
According to another aspect of the present invention, a method of making a phase change material includes the steps of providing a composition of a metal nitrate and water and adding tetraborate.
According to a further aspect of the present invention, a phase change material is made by the process including the steps of providing a composition of a Group IA metal nitrate, a Group IIA metal nitrate and water and adding a tetraborate salt.
The step of providing a composition of a Group IA metal nitrate, a Group IIA metal nitrate and water includes the step of providing a composition of lithium nitrate and hydrated magnesium nitrate. The process may also include the step of adding an effective amount of an aqueous material sufficient to cause the densities of the liquid and solid phases of said phase change material to be approximately equal during phase transformation.
Alternatively, the step of providing a composition of a Group IA metal nitrate, Group IIA metal nitrate and water includes the step of providing a composition of lithium nitrate, hydrated magnesium nitrate, and an effective amount of an aqueous material sufficient to cause the densities of the liquid and solid phases of said phase change material to be approximately equal during phase transformation.
The step of adding a tetraborate salt may include the step of adding sodium tetraborate, for example, in the form of sodium tetraborate decahydrate. In fact, the step of providing a composition of a Group IA metal nitrate, Group IIA metal nitrate and water may include the step of providing a composition of lithium nitrate and hydrated magnesium nitrate, and the step of adding a tetraborate salt may include the step of adding an effective amount of a hydrated tetraborate salt (e.g., sodium tetraborate decahydrate) such that the water added to the phase change material by the hydrated tetraborate salt is sufficient to cause the densities of the liquid and solid phases of said phase change material to be approximately equal during phase transformation.
The process may also include the step of adding a strong base, for example, lithium hydroxide, sodium hydroxide, barium hydroxide, or potassium phosphate. Further, the process may include the step of adding an oxidizer, for example, a permanganate salt (e.g., potassium permanganate) or a molybdate salt. Also, the process may include the step adding an inhibitor such as a silicate (e.g., metasilicate) or silicic acid.
According to yet another aspect of the present invention, a method of making a phase change material includes the steps of providing a composition of a Group IA metal nitrate, a Group IIA metal nitrate and water, and adding a tetraborate salt.
Preferably, the step of providing a composition of a Group IA metal nitrate, a Group IIA metal nitrate and water includes the step of providing a composition of lithium nitrate and magnesium nitrate hexahydrate.
The step of adding a tetraborate salt may include the step of adding sodium tetraborate, for example, in the form of sodium tetraborate decahydrate.
The process may also include the step of adding a strong base selected from the group consisting of lithium hydroxide, sodium hydroxide, barium hydroxide, and potassium phosphate. Similarly, the process may include the step of adding an oxidizer selected from the group consisting of permanganate salts and molybdate salts. Further, the process may include step of adding an inhibitor selected from the group consisting of silicates and silicic acid.
According to a still further aspect of the present invention, a heat battery includes a passage through which a working fluid may pass, a container in heat exchange relationship with the passage, and a phase change material disposed in the container and formed from a Group IA metal nitrate, a Group IIA metal nitrate, water and a tetraborate salt.
The phase change material may be formed from lithium nitrate, magnesium nitrate hexahydrate, and sodium tetraborate. Additionally, the phase change material may further include a strong base selected from the group consisting of lithium hydroxide, sodium hydroxide, barium hydroxide, and potassium phosphate. Also, the phase change material may further include an inhibitor selected from the group consisting of silicates and silicic acid. The phase change material may further include an oxidizer selected from the group consisting of permanganate salts and molybdate salts.